% 13 C = Percentage of tracer oxidated over the test period % 13 C-dose/h = Percentage of the dose of 13 C administered recovered per hour 13 C-peak = Time of the peak exhalation Introduction Liver mitochondria are essential for hepato- cyte function, representing the major source of energy through ATP synthesis. In particular, the function of electron transport chain was found to be impaired both in acute and chronic liver dis- eases 1 , leading to decreased oxidative metabo- lism of various substrates and to impaired recov- ery of the hepatic energy state after a metabolic insult. Interestingly, the development of hepato- cellular steatosis in chronic alcohol abusers was consequence of reduction of mitochondrial lipoperoxidations 2 . Therefore, mitochondrial function assessment might represent an impor- tant tool for the management of patients with liv- er disease. However, tests available to explore mitochondrial activity are complex and often in- vasive 3 . Breath testing is a safe and non-invasive tool for investigating the integrity of different metabolic pathways by means of 13 C-labeled sub- strates 4 . Tests based on substrates producing CO 2 during hepatic mitochondrial metabolism, such as branched-chain α-ketoacids (BCKA), could give a dynamic analysis of liver mitochondrial function and have been proposed in the last decade to assess mitochondrial function. In par- ticular, the decarboxylation of ketoisocaproic acid (Kica), occurring almost completely in he- patic mitochondria is the first step of metabolism that finally produces CO 2 3 . 13 C-Kica breath test was first proposed to assess mitochondrial func- tion in rats 5 . Then it was proposed as a specific marker of ethanol intake in chronic ethanol 401 Abstract. – Background and Aims: 13 C- Ketoisocaproic Acid Breath Test ( 13 C-Kica-BT) has been proposed to assess mitochondrial function. Aim of this study is to evaluate whether gender affects mithocondrial oxida- tion by means of 13 C-Kica-BT in healthy sub- jects in basal conditions and after an acute ox- idative stress induced by ethanol. Methodology: 50 healthy volunteers were given 1 mg/kg of 13 C-Kica together with 20 mg/kg of L-leucine dissolved in 200 ml of or- ange juice. Breath samples were taken at base- line, every 5 minutes for 45 minutes and then every 15 minutes until 2 hours. Forty-eight hours later the test was repeated 30 min after ethanol ingestion (0.5 g/kg body weight). 13 CO 2 enrichment in breath was analyzed by isotope ratio/mass spectrometry. Statistical analysis was performed using the student’s t test. Results: At baseline conditions, the percent- age of Ketoisocaproic acid in 2 hours was sig- nificantly higher in females than in males. Ethanol significantly reduces the oxidation of Ketoisocaproic acid. Conversely, no differ- ences were observed between groups after the ethanol oral load. Conclusions: Decarboxylation of 13 C-Kica was significantly higher in females than in males. Ethanol decreases Kica decarboxyla- tion in particular in women. Further studies re- main needed to establish whether sexual hor- mones could interfere with the metabolism of Kica. Key Words: Gender, Ethanol, Kica breath test. Abbreviations 13 C-Kica-BT = 13 C-Ketoisocaproic Acid Breath Test BCKA = Branched-chain α-ketoacids Kica = Ketoisocaproic acid BMI = Body mass index European Review for Medical and Pharmacological Sciences 2007; 11: 401-406 Corresponding Author: Marcello Candelli, MD; e-mail: mcandelli@gmail.com Gender affects 13 C-ketoisocaproic acid breath test M. CANDELLI 1 , A. ARMUZZI 1 , L. MIELE 1 , E.C. NISTA 1 , G. PIGNATARO 2 , L. ZILERI DAL VERME 1 , A. GRIECO 1 , G. GASBARRINI 1 , A. GASBARRINI 1 1 Internal Medicine, and 2 Emergency Departments, Catholic University of Rome (Italy)